Ok so the evil Forgotten Ones turned on to humans, made them their new believers. But didn't Solas say that Arlathan was already into shatters when the Tevinterani conquered them?They do. I wouldn't be surprised if the elven god of pestilence or the god of terror was actually the old god Urthemiel, god of beauty (last archdemon). Reminds me of the four horsemen of the apocalypse, which is rather fitting.
Now that I think about it Solas could have been lying about not tricking the elven gods. Both good and evil. And the evil ones were banished by Solas into the Deep Roads somehow. But if he tricked them why is Flemeth/Mythal not angry at him?